This application is a 371 of PCT/CA99/01205, which claims priority from Canadian Patent Application No. 2,256,989; filed Dec. 18, 1998 and U.S. Provisional Patent Application No. 60/133,249; filed May 6 1999. The disclosures of Canadian Patent Application No. 2,256,989 and U.S. Provisional Patent Application No. 60/133,249 are incorporated herein by this reference to them.
This invention relates to a process and apparatus for treating municipal, industrial, agricultural or other wastewater feeds in a submerged membrane bioreactor. Specifically, the invention concerns the removal of ammonia and total nitrogen in a single tank submerged membrane bioreactor through the processes of nitrification and denitrification.
Excessive nitrogen is often introduced into the environment through the discharge of municipal, industrial, agricultural and other wastewaters. Such wastewater feeds contain organic nitrogen primarily in the form of proteins and ammonia. During wastewater treatment, ammonia and other sources of total nitrogen are partially oxidized to produce nitrites and nitrates which can be hazardous to the x environment. For example, discharging excessive amounts of total nitrogen into rivers and lakes contributes to their eutrophication which is characterized by frequent algal blooms and reduced levels of free oxygen available to plants and fish. Recently, many jurisdictions have started to regulate the amounts of total nitrogen and ammonia that may be discharged from wastewater treatment plants.
To remove ammonia and total nitrogen from wastewater, bacteria in a bioreactor are used to alternately nitrify and denitrify the water. During nitrification, a two step process converts ammonia into nitrites and nitrates. Firstly, bacteria primarily of the genus Nitrosomonas oxidize the ammonia and convert it to nitrite. Secondly, bacteria primarily of the genus Nitrobacter oxidize the nitrite and convert it to nitrate. Since oxidation occurs in both steps of the nitrification process, an aerobic environment is required. During denitrification, nitrate is converted to ammonia and nitrogen gas by other bacteria (which could include the bacteria mentioned above after they have switched functions) and fungi which remove oxygen from the nitrate. Anoxic conditions are needed to encourage the growth of such bacteria. After denitrification, the nitrogen gas leaves the process and joins the atmosphere and the ammonia is treated by further nitrification and denitrification steps. By cycling back and forth between nitrification and denitrification steps, or aerobic and anoxic conditions in the bioreactor, nitrogen continuously leaves the water reducing the levels of total nitrogen and ammonia in the wastewater.
Submerged membrane bioreactors have been used to remove biological oxygen demand (BOD) and suspended solids from wastewater streams but they are difficult to use under anoxic conditions. A membrane bioreactor typically comprises a plurality of ultraporous or microporous membranes submerged in a tank of wastewater with suction applied to one side of the membranes. Clean water permeates through the membrane walls but bacteria and suspended solids are rejected by the membranes and remain in the tank to be biologically treated. To prevent pollutants in the tank from rapidly fouling the pores of the membranes, the membranes are typically kept awash in air bubbles. Without aeration, the membranes would quickly foul and lose their permeability, but the aeration prevents anoxic conditions and denitrification from occurring in the membrane bioreactor.
Multistage systems have been used to provide both aerobic and anoxic conditions for nitrogen treatment. In these systems, an aerated membrane bioreactor is used to provide an aerobic environment suitable for digesting BOD and nitrifying ammonia. A second bioreactor, without a membrane, provides an anoxic environment for denitrifcation and partially treated wastewater (mixed liquor) continually flows between the two tanks. These systems are undesirable in many applications because the cost and space required for the second tank and mixed liquor transfer apparatus is prohibitive.
Nitrogen has also been treated in commercial applications in a single tank membrane bioreactor where the oxygen content in the bioreactor is varied to produce alternating aerobic and anoxic conditions. In these systems, permeation through the membranes typically stops during the anoxic phase to avoid membrane fouling in the absence of membrane aeration. The loss of permeation during the anoxic phase significantly reduces the daily yield of filtered permeate through the membranes and is a disadvantage of these systems.
Japanese Patent Application No. HE I 5-220346 discusses a system combining the single tank and multistage systems described above. The system has an anoxic tank for denitrification and a membrane bioreactor for nitrification and digesting BOD with mixed liquor recirculating between them. Filtered effluent is permeated through submerged membranes filters in the membrane bioreactor. If the rate of denitrification in the anoxic tank is insufficient, the air supply to the membrane bioreactor is discontinued, to bring the tank into anoxic conditions. However, when the air supply to the membrane bioreactor is disconnected, permeation through the submerged membrane filter is also stopped to prevent membrane fouling. Again the average yield of the system is compromised.
Japanese Patent Application HI 6-181645 describes a single tank membrane bioreactor for treating nitrogen which is cycled between aerobic and anoxic conditions by alternately turning an air supply to the membranes off and on. In one embodiment, an attempt was made to continue to permeate effluent through the membranes during the anoxic phase. Each anoxic phase was started with a minimum level of mixed liquor in the bioreactor and the level of mixed liquor was allowed to rise to a maximum level by the end of the anoxic phase so that a reduced amount of permeate could be withdrawn during the anoxic phase. The yield of the system was reduced because permeate was not withdrawn at the full rate during the anoxic phase and yet it was found that the membranes still fouled quickly. In another mode of operation, aeration was again used during an aerobic period and then stopped to allow an anoxic phase to begin. Permeation through the membranes was stopped at the beginning of the anoxic phase but resumed after the mixed liquor had settled to the bottom of the bioreactor. With this method, the time needed to allow the mixed liquor to settle undesirably reduces the frequency at which the reactor can be switched between anoxic and aerobic phases and the system yield is still reduced because permeation stops during the settling period.
European Patent Application EP 0 695 722 A1 shows in its second embodiment a single tank membrane bioreactor also containing vinylidene chloride fillers. The bioreactor is continuously aerated. A layer of microoigarisms on the vinylidene chloride fillers is dispersed and broken into a fine floc every 12 hours to consume oxygen so as to create anaerobic conditions in the bioreactor.
It is an object of the invention to provide a simple and cost effective membrane bioreactor that produces a high yield of water having acceptable levels of nitrogenous compounds. In particular, it is an object of the invention to provide a singe tank membrane bioreactor which can be cycled between primarily anoxic and aerobic conditions without stopping or reducing the rate of permeation during the anoxic phase.
To accomplish these and other objects, the present invention is directed at an apparatus for treating feed water with pollutants containing nitrogen in a bioreactor having:
(a) a tank for holding mixed liquor to be treated;
(b) a feed water inlet to the tank;
(c) at least one membrane having a first side in fluid communication with mixed liquor in the tank and a second side in fluid communication with a header;
(d) a source of negative pressure to the header;
(e) a membrane scouring bubble supply; and,
(f) an oxygenating bubble supply actuatable between on and off states.
The invention is further directed at a process wherein the membrane scouring bubble supply, located below the membrane or membranes, continuously provides large scouring bubbles to clean the membranes. Treated water is Permeated through the membranes continuously at a high rate of yield but the large bubbles do not transfer sufficient oxygen to the feed water to create aerobic conditions throughout the reactor. The large scouring bubbles create an airlift effect which causes a recirculation pattern in the mixed liquor but oxygen is depleted from the mixed liquor or diluted as the mixed liquor travels away from the membranes. The concentration of dissolved oxygen is reduced in a region below or adjacent to the bottom of the membranes, and at least this region is anoxic when air is supplied by the scouring bubbles alone. If necessary, the large scouring bubbles are captured in a hood after they have scoured the membranes to provide an oxygen lean source of further scouring bubbles.
The oxygenating bubble supply is operated intermittently and provides small bubbles of air or oxygen to intermittently produce aerobic conditions in at least part of the tank. Minimum and maximum periods of aeration and non-aeration through the oxygenating bubble supply are preselected but the level of dissolved oxygen (DO) or oxygen reduction potential (ORP) in the tank is measured by sensors and assists in controlling the aerating bubble supply. In particular, a period of aeration is terminated if it is within the time limits and the level of DO or ORP in the tank exceeds a maximum value and a period of non-aeration is terminated if it is within the time limits and the level of DO or ORP in the tank drops below a minimum value.